Carbon-nanotube arrays take heat off chips

Portland, Ore. -- A carbon-nanotube-based thermal material has been crafted by researchers at Purdue University to transfer heat away from the densely packed transistors that are increasingly being crammed onto silicon chips. The researchers claim that the material transfers heat away from chips to any heat sink faster than the liquid-cooled method used by many of today's manufacturers.

"Carbon nanotubes have excellent heat conduction properties," said Timothy Fisher, an associate professor of mechanical engineering at Purdue University. "The performance that we see with nanotubes is significantly better than comparable state-of-the-art commercial materials." Fisher performed the work with his student, doctoral candidate Jun Xu.

Heat sinks traditionally use metal fins to dissipate heat into the air. Even liquid-cooling fins, which circulate water through their inside channels to an externally cooled reservoir, still need to transfer the heat from the silicon chip to the heat sink. Ordinarily, an electrical insulator like mica, which nevertheless conducts heat, is used between the silicon chip and the heat sink. A thermal-transfer paste is applied to both sides of the mica before it is inserted between the chip and heat sink.

The Purdue researchers contend that a carbon nanotube can transfer heat from the chip through the mica and to the sink more that three times more efficiently than paste.

Like Velcro"We have seen a strong synergistic effect by using a combination of nanotube material and traditional interface materials," said Fisher. "The two pieces come together in such a way that they facilitate heat flow, becoming the thermal equivalent of Velcro."

Under the microscope, the nanotube-based materials look like carpets of tiny cylinders, each of which enhances the flow of heat away from chips. They fill the gaps between the chips and heat sinks that are normally occupied by mica and paste.

The researchers have experimented with several varieties of nanotube-based thermally conducting materials, but thus far they have found that the Velcro-like nanocarpet approach produces the best results. The carbon nanotube arrays provide a thermal interface that conducts heat several times more efficiently than conventional thermal interface materials at the same temperatures, according to Fisher and Xu.

"We coat both sides of the interface with nanotubes, which doesn't create a strong mechanical bond but greatly facilitates heat flow," said Fisher.

Fisher and Xu described their results in the May issue of the International Journal of Heat and Mass Transfer. There they compared their nanotube-based materials with traditional thermal interface materials and with several combinations of the two. Dry CNT arrays produce a thermal interface resistance of 19.8 square millimeters Kelvin per watt. The combination of a CNT array and a phase-change material produced an even lower minimum resistance, of 5.2 square mm K/W.

Nanotubes provide ballistic transport of electrons by offering almost no resistance as they travel from one end to the other. Likewise, nanotubes transfer heat with much less thermal resistance. A typical mica- and paste-based heat sink will rise in temperature about 15°C when a microprocessor or power transistor starts heating up. But the Purdue researchers claim their nanotube-based material only rises in temperature about 5°, effectively transferring three times as much heat across to the sink.

The carbon nanotube arrays were synthesized directly on silicon wafers using plasma-enhanced chemical vapor deposition. They were tested on a one-dimensional reference bar in a high vacuum with radiation shielding. Temperatures were measured with an infrared camera.

The researchers used nanotubes whose diameters varied in size, ranging from single-walled diameters of less than 1 nanometer to multiwalled diameters of more than 100 nm. They are still working to achieve just the right combination of sizes for specific heat sink application.

The material is being developed for commercial applications by Nanoconduction Inc. (Sunnyvale, Calif.). In addition to Nanoconduction, several other commercial and military contractors are cooperating with the Purdue researchers to craft formulations for specific applications in computer, telecommunications and power electronics for aircraft, ships and vehicles.

The research was funded by Purdue's Cooling Technologies Research Center, of which Nanoconduction is a member, as well as the National Science Foundation. The researchers are also cooperating with colleagues at the Air Force Research Laboratory and the Birck Nanotechnology Center at Purdue's Discovery Park.